NetBSD is the more portable one.

FreeBSD is maybe the best well rounded, has zfs.

OpenBSD is all about top notch security by default.

Generally the documentation for them is accurate. If the manpage says something should work it will work. The components are more tightly integrated than a gnu/linux system since they’re all maintained together.

I have long wished they would find some killer application(s) for the server sector and grow in widespread usage. (And I have a long held commitment to myself to support them significantly, financially if I ever win a lotto ;) Their licensing is much easier for a corporation to use than linux and there are products which are based on them.

They probably would work well as a backend OS as they’re known for stability.